Spread spectrum communication systems provide significant improvements in communication systems which must operate in noisy environments. Consider a simple digital communication system in which messages consisting of strings of bits are sent from a transmitter to a receiver. The transmission typically involves modulating a carrier signal with a waveform which specifies the value of the bit being sent. The output of the transmitter for a single bit will be referred to as the bit signal in the following discussion. In a binary transmission system, there are two such bit signals, one representing zero and the other representing one.
Upon reaching the receiver, the detected bit signals are compared with those corresponding to one and zero to ascertain each bit in the sequence. In a noisy environment, any bit signal may be corrupted by a noise pulse in its journey from the transmitter to the receiver. The probability of such a corruption is related to the statistical distribution of the noise pulses. In particular, it is related to the probability of finding a noise pulse which will add sufficient energy to the bit signal during transmission to corrupt that bit in a manner that will cause the receiver to mis-identify the bit signal.
It should be noted that the addition of a constant amount of energy to each bit signal would cause no corruption of the signal, as the receiver could compensate by subtracting a constant from each detected bit signal. Hence, the corruption results from adding different amounts of noise energy to different bit signals. The variation in the amount of energy added to each bit signal will depend on the duration of the bit signals. If the bit signals are long compared to the duration of the noise pulses, than the energy added to any given bit signal will be the average of the energies in a large number of noise pulses. Such an average will be relatively constant from bit signal to bit signal. If, however, the duration of the bit signals is of the same order as that of the noise pulses, the variation in added noise energy from bit signal to bit signal will be relatively large. Hence, it is advantageous to use relatively long bit signals.
If the bit signals did not to overlap in time, such a strategy would result in a very low transmission bit rate on the communication channel. Such a reduction in transmission rate is clearly to be avoided. Hence, a system in which the bit signals from adjacent bits in the message must overlap is desirable. The resultant transmission at any given time is the sum of the bit signals from a large number of bits. Such a system will be referred to as a spread spectrum communication system in the following discussion.
Consider a simple system in which the bit signal for zero consists of the sequence G[0,i] for i=0. . . L-1, and the bit signal for one consists of the sequence G[1,i] for i=0. . . L-1. In the following discussion, "L" is generally used for the number of elements in the bit signal. A sequence of bits b(j) is to be sent. It is assumed that the transmitter and receiver are clocked at a predetermined rate and that a bit is introduced into the apparatus on each clock pulse. The transmitter includes L shift registers, each of length L. When a bit is to be transmitted, the appropriate bit signal is loaded into the next free shift register. On each clock pulse, all of the shift registers are shifted once, and the data which is shifted off of the end of each shift register is added to form the transmitted signal. The signal will then be a sequence .sigma.(k), where k denotes the clock pulse and ##EQU1## In the absence of noise, the bits b(j) can be recovered at the receiver by solving an L.times.L system of linear equations. However, the computational workload involved makes this approach impractical for long waveforms.
If, however, the bit signals are orthogonal, the matrix representing the system of linear equations is diagonal, and the bits can be recovered by computing the correlation of the signal .sigma.(k) with the sequences G[1,i] and G[0,i]. If a bit having a value of B was sent at starting at time K, then the correlation function ##EQU2## should be one for b=B and 0 otherwise. It should be noted that the orthogonality requirement restricts the manner in which the message is sent. In particular, it can be shown that no finite sequence G[b,i] is orthogonal to itself shifted by one. Hence, if orthogonal sequences are utilized, the data bits can, at most, be introduced into the transmitter on every other clock pulse.
Orthonormal bit signals have the additional property of being more immune to burst noise for a given length waveform. If non-orthonormal signals are used and .sigma.(k) is corrupted by a burst of noise at some time K, sufficient information may be lost to prevent the system of equations from being solved. If, however, orthonormal bit signals are utilized, the effect of the noise is to lower the correlation value for the correct bit value and increase the correlation for the incorrect bit value. Hence, provided the noise is not sufficient to make the correlation values corresponding to different bits indistinguishable, the signal can be recovered even in the presence of burst noise. Hence, orthonormal bit signals are preferred.
In addition to orthonormality, it is advantageous to utilize bit signals which have the property that the sum signal .sigma.(k) is statistically indistinguishable from the noise to within some tolerance. It can be shown that this requirement allows the system to utilize a greater fraction of the bandwidth of a noise channel for communication (C. Shannon "Communications theory of secrecy systems", Bell Systems Tech. Journal, 28 pp. 676-715, 1949).
This last property also provides a means for embedding a spread spectrum communication in the noise in a communication channel being used for other purposes. In addition to providing security for the information being transmitted, such a system may also be used to multiplex communications. For example, a spread spectrum system satisfying this condition could be used to send digital data on top of voice data in a telecommunication system. The digital data would merely appear as low level random noise. The extent to which background interference is found to be objectionable to human listeners depends greatly on the form of the interference. In particular, human listeners are less sensitive to random noise than to noise having a regular pattern. Hence, a significantly higher signal power may be used for the digital communication if the spread spectrum system is indistinguishable from the typical static encountered on telephone lines.
While the basic features for such a system have been known for some time, no practical system meeting all of the conditions has been provided in the prior art. For example, Shannon proposed using two sequences, G.sub.0 and G.sub.1, for the bit signals in which each sequence consists of numbers having a Gaussian random distribution. It may be shown that two random sequences are statistically orthonormal.
Unfortunately, this suggested method has a number of problems. It should be noted the it is advantageous to be able to vary the length of the bit signals in response to the noise environment. When used to provide a reduction in noise errors, there is an optimal length for the bit signals. The computational load inherent in the correlation operation for decoding the signal at the receiver is related to the length of the bit signal. Hence, shorter bit signals are preferred. On the other hand, the ability to withstand noise interference improves with the length of the bit signal. However, there is a point at which doubling the length of the bit sequence results in only a small improvement in interference rejection. If the bit signal is much longer in time than the average noise pulse, very little additional improvement is obtained by further increasing the size of the bit signal.
It is impossible to generate truly random signals of finite length. Pseudo-random sequences are known to the prior art; however, to satisfy the orthonormality conditions within a sufficient tolerance, the length of a pseudo-random sequence must be quite long. Hence, bit signals which are much longer than the optimum length must often be used if pseudo-random sequences are employed. Ideally one would like to be able to vary the length of the bit signals without altering the orthonormal character of the signals. No practical method for generating specified length sequences that appear random having this property is taught in the prior art.
In addition to the above mentioned problems, prior art systems are difficult to synchronize. Before a receiver can interpret the message sent by a transmitter, the transmitter and receiver must be synchronized. Initial synchronization is the most difficult to achieve. In long data transmissions, the receiver and transmitter must be periodically re-synchronized to correct for small differences in the frequencies of the clocks used in each device and/or drifts in the clock frequencies. In addition, changes in the distance between the transmitter and receiver can also lead to a loss of synchronization. Such changes in distance are encountered when one of the parties is utilizing mobile communications equipment such as a cellular telephone.
To achieve initial synchronization without using some form of non-spread spectrum encoding, the phase of the receiver's clock is typically varied until a synchronization message is successfully decoded. The time needed to achieve initial sychronization is related to the length of the synchronization message and to the length of the bit signals used in the spread spectrum code. Hence, short synchronization messages and codes are preferred. However, the synchronization message must be long enough to guarantee that part of a valid message is not mistaken for a synchronization signal during the periodic resynchronization procedures.
If non-spread spectrum synchronization signals are used, the power levels must be increased which can cause interference with other signals in the communication band. Such high power signals can be objectionable if the communication channel is being shared, as these signals would interfere with the other communications on the channel. In addition, in secure communication situations in which the user wish to hide the fact that a communication is taking place, such high level signals can compromise the secrecy of the communication.
Broadly, it is an object of the present invention to provide an improved spread spectrum communication system.
It is a further object of the present invention to provide a spread spectrum communication system utilizing orthonormal bit signals.
It is yet another object of the present invention to provide a spread spectrum communication system in which shorter synchronization signals than those used in prior art systems can be communicated without using non-spread spectrum codes and without being mistakenly interpreted by the receiver as part of a message.
These and other objects of the present invention will be apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.